Method for determining the mass center of two-AXLE vehicles

Abstract

The invention relates to a method for determining the mass center (S) of two-axle vehicles. When performing the method, the total mass (G) and the axle loads (Ge, Gh) of the vehicle (1) are measured, the longitudinal and transverse coordinates (Sh, Sk) of the mass center (S) are calculated on the basis of the wheelbase (1) and the track width (b), and then the height coordinate (Sm) is determined by lifting one of the axles of the vehicle (1) to a specified height (em), taking into account the lifting angle (α) and the changes in axle load (Gh2). The method includes the calculation of the static radius (R) of the wheel (2), which is given by the radius (r) of the rim (2a) and the height of the tyre (2c) with consideration of its flattening. The invention applies a software that automatically calculates and displays the position of the mass center (S) in graphical form on the basis of the measured and determined parameters. The method is particularly applicable to electric vehicles.

Description

[0001] METHOD FOR DETERMINING THE MASS CENTER OF TWO-AXLE VEHICLES

[0002] The invention relates to a method for determination of the mass center of vehicles, in particular electric vehicles. The method uses the axle load calculation, which calculates the three-dimensional position of the mass center on the basis of the load acting on the front and rear axles, the wheelbase and track width of the vehicle as well as the lifting angle and the change in rear axle load. The method is particularly advantageous in case of electric vehicles because the absence of disturbing factors arising from fluid dynamics enables more accurate calculations in case of vehicles of this type.

[0003] Determination of the mass center of vehicles is essential from the point of view of stability, handling and safety. The position of the mass center determines how the vehicle responds in different driving situations, for example when cornering, accelerating, or braking. In case of higher mass center, the rollover tendency of the vehicle increases, especially in sharp corners or during swerves, while a lower mass center improves stability and handling.

[0004] The position of the mass center affects the distribution of load between the axles of the vehicle, which in turn affects braking and acceleration performance. A properly positioned mass center ensures optimal grip of the wheels, reducing the risk of skidding or loss of grip. In addition, uniform weight distribution mitigates tyre wear and improves energy efficiency.

[0005] Determination of the mass center is particularly important in case of electric vehicles because their batteries are components of significant mass whose location directly affects the dynamic characteristics of the vehicle. In vehicles equipped with electric powertrains, the disturbing factors arising from fluid dynamics, such as fuel motion, do not occur, so the mass center can be calculated more acccurately and reliably. This accuracy is essential for optimizing vehicle stability, especially when designing and testing new-generation electric vehicles.

[0006] Several methods are used for determination of the mass center of vehicles. One of the most common methods is the component-based calculation, which takes into account the mass and spatial position of each component of the vehicle. The axle load calculation method is also frequently used, which offers a simpler and faster solution. This method calculates the position of the mass center on the basis of the loads acting on the axles of the vehicle as well as the wheelbase and track width. A similarly widespread method is the longitudinal lifting method, where the mass center height is calculated on the basis of the angle and the change in axle loads created by lifting the vehicle. Under laboratory conditions, dynamic measurement methods are also used, which utilize the data originating from the vehicle motion. The latter methods are based on data recorded by acceleration and rotation sensors by the aid of which the position of the mass center of the vehicle can be accurately determined. The simple pendulum method is based on the oscillation of the vehicle about a single suspension point. The mass center height is calculated on the basis of the period and the arc length of the oscillation. The suspension method is a further prevalent one, wherein the vehicle or object is fixed at various points, and the position of the mass center is determined on the basis of their location. The method is suitable for vehicles of larger dimension.

[0007] The patent document CN112393845A, which is closest to the invention, discloses an improved version of the longitudinal lifting method for determination of the mass center height of two-axle vehicles. The method takes into account the main parameters of the vehicle, including wheelbase, unladen weight, front and rear suspension stiffness, static wheel radius as well as axle loads. The solution aims at improving the accuracy of determination of the mass center height by taking into account the deformation of the tyres and the change in suspension caused by the redistribution of load when lifting the vehicle.

[0008] The utility model CN203148627U discloses a solution wherein the mass center height of the vehicle is measured under dynamic conditions in real traffic situation. While moving, the vehicle rolls over the scales built into the road surface, which measure the axle loads before and during braking. These data together with deceleration values recorded by an accelerometer are processed by a control unit. The results are calculated using the principle of moment equilibrium, and the system determines the mass center height of the vehicle on the basis of the available data.

[0009] The solution disclosed in patent document JP2012083194A describes a device and a method that measures the mass center height of an object, for example of a vehicle, on the basis of the principle of the physical pendulum. The device comprises a swinging mechanism that moves the object in a curved path and ensures the stable movement by rollers or a curved guide. Sensors are used to measure the period of the oscillation from which the mass center height is calculated using a mathematical formula. The design of the device simplifies the measurement because it requires no suspension, thus it can be safely used in case of objects of larger mass, too.

[0010] Previous solutions for determination of the mass center of vehicles have several limitations and difficulties. The component-based calculation provides high accuracy, but it is extremely time-consuming and requires detailed data about each component of the vehicle. Solutions that take into account suspension stiffness and tyre deformation during lifting are more accurate, but they are time-consuming and resource-demanding because of their complexity. Measurements taken under dynamic conditions, such as using scales built into the road surface, can only be applied in traffic situations and often result in inaccuracies due to road quality, vehicle speed or accelerometer calibration. The disadvantage of the swinging mechanism method is that the accuracy of the measurement depends largely on the precise design of the curved path and the rollers or curved guides; furthermore, the space requirement and complexity of the device limit its applicability. In addition, the above methods are generally not specialized for a particular type of vehicle, so their use is not always optimal.

[0011] The invention aims to improve the longitudinal lifting method in such a way that it enables accurate, fast and reliable determination of the mass center of vehicles. An object of the invention is to introduce a lifting height that is safe and universally applicable to various electric vehicles, while it provides sufficient accuracy for precise calculation of the height coordinate. Further objectives include specialization of the method in such a way that it takes into account the structural characteristics of vehicles, which enables to have more precise results. Finally, an object of the invention is to use a software that automatically calculates and graphically displays the position of the mass center of the vehicle on the basis of the specified parameters, thereby speeding up and simplifying the calculation process while minimizing the possibility of human error.

[0012] The inventors have discovered that, according to the state of the art, the height coordinate of the mass center of the vehicle is conventionally determined at a greater lifting height because, under such circumstances, the magnitude of axle load change alone is sufficient for accurate calculation. However, such solutions carry safety and structural risks because lifting of the vehicle to a greater height may lead to instability and structural load.

[0013] The inventors have further discovered that by limiting the lifting height to a range from 10 to 18 cm, the measurement becomes safer and more universal, while the load change resulting from the lower lifting height is more moderate, therefore the calculation becomes more sensitive to minor deviations regarding the wheel geometry.

[0014] The inventors have further discovered that, within this smaller lifting range, the actual flattening of the tyre — which has been considered a negligible factor up to now when determining the static wheel radius — becomes a significant source of measurement errors. In other words, at low lifting heights, the effect of tyre deformation increases proportionally and it may considerably distort the calculated mass center value.

[0015] The inventors have also discovered that, to eliminate this, the static radius of the wheel can be determined taking into account the actual flattening of the tyre under load. This purpose is served by the introduction of the correction factor (k), which determines the tyre height not as a nominal but as a corrected value, depending on the load conditions.

[0016] As an outcome of these discoveries, it has become clear to the inventors that the combined use of the low lifting height (10 to 18 cm) and the correction factor (k) creates a synergistic effect that would not be achievable by using either factor alone. The low lifting height alone would not ensure the sufficient accuracy, the use of the correction factor alone is not justified at great lifting heights, but the combination of the two enables a safe, fast and, at the same time, highly accurate determination.

[0017] The technical effects appear on several levels accordingly: the accuracy of the calculation improves, the measurement becomes safer and the method is universally applicable in the environments of both series production and development, where liftings of great heights are difficult to perform.

[0018] The inventors have further discovered that, in case of electric vehicles, the determination of the mass center can be more accurate than in case of vehicles equipped with internal combustion engines because dynamic factors, such as movement of liquid in the fuel tank, which may have a significant impact on the weight distribution, are lacking.

[0019] The essence of the method is that the axle load method is used for determination of the mass center of the vehicle, taking into account the loads acting on the front and rear axles of the vehicle, the wheelbase, the track width and the lifting angle. When using the method, the vehicle is lifted to a predetermined safe lifting height that is sufficient to produce the necessary load changes. Based on the measured data, the longitudinal, transverse and height positions of the mass center can be calculated. The method specially considers the static radius of the wheel, which is determined on the basis of the rim radius and the tyre height. This increases the accuracy of the calculations, especially when determining the height coordinate. The measurement results are processed by a software that performs not only the calculations but also displays the position of the mass center in graphical form, thereby making the method simpler and faster.

[0020] The drawings illustrating the operation of the method contain the following elements:

[0021] Figure 1 : Longitudinal position and height of the mass center of the vehicle with indication of the wheelbase.

[0022] Figure 2: Transverse position of the mass center of the vehicle with indication of the track width. Figure 3: Illustration of the lifting procedure, showing the lifting of the vehicle to a specified height and the resulting lifting angle.

[0023] Figure 4: Determination of the static radius of the tyre with the wheel radius calculated on the basis of the relationship with the rim radius and the tyre height.

[0024] Figure 5: Illustration of the software operation, showing an exemplary display of the input data and the graphical results.

[0025] When using the method, the three-dimensional position of the mass center (S) of the vehicle 1 is determined on the basis of the axle loads, the geometric parameters of the vehicle 1, the lifting height, and the static radius of the wheel 2. In order to facilitate the interpretation, the symbols used in the calculations are summarized in the table below.

[0026] In a first step of the method, the total mass of the vehicle 1 and the axle loads are measured by means of a high-precision vehicle platform scale. The wheelbase and the track width are determined on the basis of the parameters specified by the manufacturer or, if necessary, through measurement. These basic data are required to calculate the position of the mass center. The longitudinal coordinate of the mass center is calculated by the following formula:

[0027] The result represents the distance of the mass center from the front axle (Figure 1). For determination of the transverse coordinate, the sum of right side components of the axle loads is divided by the total mass and then multiplied by the track width:

[0028] This result gives the lateral position of the mass center relative to the left tyre centerline of the vehicle 1 (Figure 2). For determination of the height coordinate, the vehicle 1 is first lifted to a specified lifting height (Figure 3). Lifting of the vehicle 1 can be performed using a hydraulic lifting platform, a two-post lift or any other lifting system that provides stable support. Due to the lifting, the rear axle load changes, which is recorded during the measurements. The height coordinate of the mass center is calculated taking into account the lifting angle and the change in rear axle load (Gh2) resulting from the lifting: ctg(a) + R where the lifting angle (a) is defined on the basis of the predetermined lifting height and the wheelbase:

[0029] It is to be noted that the expression ctg(cr) can be replaced directly by the ratio of the wheelbase and the lifting heigth, too. The choice of lifting height plays a key role in accurate determination of the height coordinate of the mass center. Lower lifting heights, such as 1 cm, do not cause sufficient load changes, therefore the calculations may be inaccurate, while liftings of excessively great heights - for example, above 30 cm - may endanger the structural integrity of the vehicle. The reason is that standard vehicles do not usually have special lifting points that would enable the safe lifting of the vehicle to a greater height without causing damage. The optimal lifting height is in the range between 10 and 18 cm, it is preferably 14 cm, because this provides adequate load change for accurate calculations while maintaining the stability and integrity of the vehicle 1. When calculating the height coordinate, the static radius of the wheel 2 is considered, which is obtained as the sum of the radius of the rim 2a and the height of the tyre 2c (Figure 4):

[0030] R = r + Am

[0031] The height Am of the tyre 2c is determined by the following equation:

[0032] Am=w-p-k where w is the width of the tyre 2b, p is the profile ratio and k is a correction factor that takes into account the actual flattening of the tyre under load.

[0033] In a possible exemplary embodiment, the value of the correction factor k is preferably determined as a function of the measured tyre pressure and / or the temperature of the tyre 2b. This enables the value of Am to be dynamically adjusted to the real operating conditions, thus making the calculation of the height coordinate Sm of the mass center S more accurate.

[0034] The value of the correction factor k is preferably in the range between 0.90 and 0.97, it is more preferably 0.95. The use of this range ensures that the deformation of the tyre 2b under load can be realistically taken into account in the calculation.

[0035] For example with a tyre of 185 / 65 R14 dimensions:

[0036] Am= 185 • 0,65 • 0,95 « 114 [mm]

[0037] In the calculations, this approach enables the use of the actual dimension of the wheel 2 under load, which more accurately reflects the real geometrical conditions, thus the determination of the height coordinate becomes more precise.

[0038] The collected data and the calculations are processed by a special software that automatically calculates the longitudinal, transverse and height coordinates of the mass center of the vehicle 1. The software displays the results in graphical form (Figure 5), thus simplifying their interpretation and minimizing the possibility of human error. After the input parameters have been entered, such as axle loads, wheelbase, track width, lifting height as well as width, profile ratio and pressure of the tyre, the software automatically calculates the longitudinal (Sh), transverse (Sk) and height (Sm) coordinates of the mass center. The results of the processing are displayed in graphical form, which clearly illustrates the spatial position of the mass center of the given vehicle.

[0039] It is to be noted that the solution is not limited to the method described and derived above because the front axle load can also be used instead of the rear axle load for determination of the longitudinal coordinate of the mass center. Similarly, the left side mass can also be used instead of the right side mass for determination of the transverse coordinate. As for the determination of the height coordinate of the vehicle 1, not only its front but also its rear can be lifted, which can also provide suitable data for the calculations.

[0040] The method can be widely used in the course of design, manufacture and testing of vehicles. The precisely determined mass center enables the optimization of stability and dynamic behavior characteristics of the vehicle, which is essential for safe and efficient operation. In the manufacturing process, the method can be used to check whether the actual weight distribution of the vehicles meets the design parameters, thus ensuring compliance with the quality requirements. The method can also be used in the development of prototypes, where the determination of the mass center assists in fine-tuning the wheel geometry and the dynamic behavior characteristics of the vehicle. In addition, the method can also be integrated into series production to meet the requirements regarding the stability and handling of the vehicles rolling off the production line. The method is particularly suitable for electric cars because their structural design has a favourable effect on the determination of the mass center. The absence of fuel tank eliminates the load changes caused by the liquid inside, which influence the position of the mass center as a dynamic factor in case of vehicles equipped with internal combustion engines.

Claims

CLAIMS1. A method for determining the mass center (S) of a vehicle (1), wherein the loads acting on each wheel (2) are measured using a vehicle platform scale, and the total mass (G) of the vehicle (1) is determined; the wheelbase (1) and track width (b) of the vehicle (1) are determined; the longitudinal coordinate (Sh) of the mass center (S) is calculated from the ratio of the load acting on one of the axles of the vehicle (1), in particular the load acting on the front axle (Ge) or the load acting on the rear axle (Gh), to the total mass (G) and from the wheelbase (1); the transverse coordinate (Sk) of the mass center (S) is calculated from the ratio of the loads acting on the wheels (2) on one side to the total mass (G) and from the track width (b); characterized in that- for determination of the height coordinate (Sm) of the mass center (S), the front or rear of the vehicle (1) is lifted to a height (em) of between 10 and 18 cm,- a lifting angle (a) is determined on the basis of the lifting height (em) and the wheelbase (1),- a height (Am) of the tyre (2c) on the non-lifted axle is determined in such a way that Am = w • p • k, where w is the width of the tyre (2c), p is its profile ratio, and k is a predetermined correction factor,- the static radius (R) of the wheel (2) on the non-lifted axle is determined as the sum of the rim radius (r) and the height (Am) of the tyre (2c), and- the height coordinate (Sm) of the mass center is calculated using the following formula, taking into account the lifting angle (a) and the change in axle load (Gh2) on the non-lifted axle: ctg(a) + R.

2. The method of claim 1, characterized in that the value of the correction factor (k) is determined on the basis of the measured tyre pressure and / or tyre temperature.

3. The method of claim 1 or 2, characterized in that the value of the correction factor (k) is in the range from 0.90 to 0.97, it is preferably 0.95.

4. The method of any one of claims 1 to 3, characterized in that, for determination of the height coordinate (Sm) of the mass center (S), the front or rear of the vehicle (1) is lifted to a lifting height (em) of 14 cm.

5. The method of any one of claims 1 to 4, characterized in that the longitudinal (Sh), transverse (Sk) and height (Sm) coordinates of the mass center are calculated using a software which, after the measured and determined parameters have been entered, displays the coordinates in graphical form.

6. The method of any one of claims 1 to 5, characterized in that, it is used for electric vehicles.

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